1. Field of the Invention
The invention relates to a filter device for a print head in an ink jet printer and, more particularly, to a filter device disposed on the way of an ink supply passage, for removing foreign materials and bubbles contained in ink.
2. Description of the Related Art
Heretofore, there have been devised print heads in an ink jet printer that are provided with a filter device for removing foreign materials and bubbles contained in ink. First, the schematic arrangement of such a conventional print head 100 will be explained below referring to FIG. 4, which is a longitudinal sectional view of a print head with a conventional filter device mounted thereon.
The print head 100 is formed mainly of a frame 101 made of magnesium alloy, which is light in weight and excellent in thermal conductivity, and is principally composed of an ink ejection unit 102, an ink reservoir 107, and an ink passage unit 114 interposed between the former two. The ink passage unit 114 and the ink reservoir 107 are separated from each other via a partition wall 101A extending downward in substantially the center of the frame 101, while they are communicated at the respective lower portions thereof with each other via a lower port 107A.
The ink reservoir 107 is provided with an air vent 109 opened to the atmosphere at the upper portion thereof, and an ink supply port 108 opening to the side at the upper portion thereof. A filter device 121 is incorporated inside the ink reservoir 107 in such a manner as to divide it into two parts. The volume of the ink reservoir 107 is about 2 cm.sup.3 As described above, the ink passage unit 114 and the ink reservoir 107 communicate at their respective lower portions with each other via the lower port 107A.
The ink passage unit 114 is constituted of an ink passage 112, a pair of negative pressure chambers 116A and 116B, and a couple of permeable thin films 120A and 120B.
The ink passage 112 is defined by the pair of permeable thin films 120A and 120B and the side wall of the frame 101. The ink passage 112 communicates at its lower portion with the ink reservoir 107 via the lower port 107A. The negative pressure chamber 116A is made up of one permeable thin film 120A, the partition wall 101A, and the side wall of the frame 101; and the negative pressure chamber 116B, of the other permeable thin film 120B and the outer wall 101B of the frame 101. The negative pressure chambers 116A and 116B are connected to a negative pressure generator, not shown, via a tube, not illustrated. The negative pressure generator is capable of keeping a predetermined negative pressure inside the negative pressure chambers 116A and 116B. The ink passage 112 communicates at its upper portion with an ink ejecting nozzle 106 of the ink ejection unit 102 through an ink inlet 102A.
The plurality of ink ejection units 102 are disposed according to the resolution of the ink jet printer. Each ink ejection unit 102 is composed of a piezoelectric vibrating element 103, an ink ejecting chamber 104, and the ink ejecting nozzle 106. The ink ejecting nozzle 106, which is made of 42-alloy (an alloy of iron mixed with 42% of nickel), is fixed to the frame 101. The piezoelectric vibrating element 103 and the ink ejecting chamber 104 are provided sideways in the ink ejecting nozzle 106 in one-to-one correspondence to the ink ejection unit 102. To the tip of the ink ejecting nozzle 106 is secured an orifice plate 110 made of nickel with orifices (ejecting ports) 110A each having a diameter of 50 .mu.m formed at predetermined intervals (i.e., at the same intervals as those of the ink ejecting nozzles 106).
In the print head 100 structured as above, the numerous piezoelectric vibrating elements 103 are selectively vibrated on the basis of an image signal. Then, ink 105 supplied to the ink ejecting chamber 104 is selectively ejected in drops toward a recording paper, not shown, through the orifices 110A formed at the tip of the ink ejecting nozzle 106. Accordingly, an image can be recorded on the recording paper with the ink 105. Foreign materials, such as dust, which is contained in the ink 105 supplied from the ink supply port 108, can be caught and removed when the ink 105 passes through the filter device 121A from left to right as viewed in FIG. 4. The clean ink 105 without any foreign material is further supplied from the right chamber in the ink reservoir 107 to the ink ejection unit 102 through the lower port 107A and the ink passage 112. On the way, air (fine bubbles) contained in the ink 105 is sucked toward the pair of permeable thin films 120A and 120B by a negative pressure is the negative pressure chambers 116A and 116B, and further, is discharged to the negative pressure chambers 116A and 116B through the pair of permeable thin films 120A and 120B.
However, since the fine filter device 121 for catching and removing the dust contained in the ink is disposed inside the ink reservoir 107 in the above described print head 100, the following problems have arisen depending on the positioning of the filter device 121.
Three possible placements of the filter device 121 are discussed below:
(1) In the case where a filter device 121A is disposed near the ink supply port 108 and inside the ink reservoir 107 as shown by the solid line in FIG. 4, as a matter of course, a volume of the ink reservoir 107 on the left side of the filter device 121A becomes smaller. As a result, although an ink supply amount to the ink reservoir 107 is 1-2 cm.sup.3 at a time, there may occur a possibility that the ink supply amount supplied cannot keep up with the rate of ink consumption in the ink ejection unit 102 since the volume of the ink reservoir 107 on the left side of the filter device 121A is small and a speed of the ink 105 passing through the filter device 121A is low. In the case where an ink amount supplied is smaller than the ink consumption, the ink amount contained inside the ink reservoir 107 on the left side of the filter device 121A becomes small so that bubbles are likely to be contained in the ink 105 in the ink reservoir 107 on the right side. Consequently, the ink 105 cannot be ejected in the ink ejection unit 102. Moreover, if the ink 105 is supplied to the ink reservoir 107 through the ink supply port 108 under large pressure in order to increase the speed of the ink 105 passing through the filter device 121A, a device for applying the pressure to the ink 105 must be provided with the disadvantages of a high production cost and a large size.
(2) In the case where a filter device 121B is disposed on the right side inside the ink reservoir 107, as shown by the chain line in FIG. 4, as the volume of the ink reservoir 107 on the left side of the filter device 121B becomes larger, newly supplied ink 105 can be easily stored in the ink reservoir 107 on the left side, thus dispensing with pressure needed for supplying the ink 105, which is necessary in the above case (1). However, in the case where the ink consumption in the ink ejection unit 102 is large, the amount of the ink passing through the filter device 121B is insufficient so that the ink amount (after filtration) stored in the ink reservoir 107 on the right side of the filter device 121B is decreased rapidly. Then, air (bubbles) is sucked into the ink passage 112 through the lower port 107A of the ink reservoir 107, with insufficient suction of the air (removal of bubbles) into the negative pressure chambers 116A and 116B through the permeable thin films 120A and 120B. As a result, the bubbles remain inside the ink ejection unit 102, hence producing a result that the ink cannot be ejected.
(3) In the case where a filter device 121C is disposed so as to vertically divide the ink reservoir 107, as shown by the two-dot chain line in FIG. 4, as the filter device 121C is fine as described previously, air contained in the ink 105 stays in a form of large bubbles under the filter device 121C. Consequently, the ink passing area of the filter device 121C becomes smaller, whereby the ink supply function is deteriorated. In the worst case, the whole lower surface of the filter device 121C is covered with the bubbles so that the ink cannot be passed through and subsequently be ejected.